git: raise hard version to 1.9.1

Debian 7 Wheezy went EOL in May 2018.  We don't need to carry support
for that anymore as there have been 5 major releases since.  Ubuntu
Precise went EOL in Apr 2019 (including the extended support phase).
That means we can bump the required git version from 1.7.9 to 1.9.1.

git-1.7.9 was released in 2012 while git-1.9.1 was released in 2014.
So that shouldn't be a problem either.  And we've been warning people
using git versions older than 1.9.1 for 3 years now that they need to
upgrade.

Change-Id: Ifbbf72f51010b0a944c2785895d1b605333f9146
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/415637
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

requirements.json

@@ -46,8 +46,6 @@
 
   # Supported git versions.
   #
-  # git-1.7.9 is in Ubuntu Precise.
-  # git-1.7.10 is in Debian Wheezy.
   # git-1.9.1 is in Ubuntu Trusty.
   # git-2.1.4 is in Debian Jessie.
   # git-2.7.4 is in Ubuntu Xenial.
@@ -55,7 +53,7 @@
   # git-2.17.0 is in Ubuntu Bionic.
   # git-2.20.1 is in Debian Buster.
   "git": {
-    "hard": [1, 7, 9],
+    "hard": [1, 9, 1],
     "soft": [2, 7, 4]
   }
 }